Proposal: Add foldrWithIndex and foldlWithIndex to Data.List

[David Feuer]

I think the answer is almost certainly no. The zipWith will turn into a
foldr2, and there's no vaguely sure way of snatching that before it fuses
with a build form and is lost forever. You'd end up with some very
complicated rules that only did something useful when the phase of the moon
was right. I'm pretty sure it's not worth trying.
On Oct 22, 2014 4:40 PM, "Ganesh Sittampalam" <ganesh at earth.li> wrote:

> I see, thanks. Could this be done via a rewrite rule from that idiom to
> an internal implementation function instead?
>
> On 22/10/2014 20:19, David Feuer wrote:
> > Yes, they do. In particular, the zip can only fuse with one of the two
> > lists so the Ints could be unboxed, or fusion optimizations could happen
> > with the list folded over, but not both. The fold_WithIndex function can
> > manage both at once. That said, I think there have been some pretty good
> > arguments against adding these, or at least against adding them with
> > these names.
> >
> > On Oct 22, 2014 3:13 PM, "Ganesh Sittampalam" <ganesh at earth.li
> > <mailto:ganesh at earth.li>> wrote:
> >
> >     On 16/10/2014 18:14, David Feuer wrote:
> >
> >         These functions can be lifted pretty much straight out of
> >         Data.Sequence.
> >         In particular, foldrWithIndex makes for a particularly nice
> >         expression
> >         of a fusing findIndices function, as is present in Data.Sequence.
> >
> >
> >     Do these do anything better than just adding indicies first with the
> >     standard zip [0..] idiom?
> >
> >     Cheers,
> >
> >     Ganesh
> >
> >     _________________________________________________
> >     Libraries mailing list
> >     Libraries at haskell.org <mailto:Libraries at haskell.org>
> >     http://www.haskell.org/__mailman/listinfo/libraries
> >     <http://www.haskell.org/mailman/listinfo/libraries>
> >
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/libraries/attachments/20141022/bf54808e/attachment.html>